It’s a Friday morning in Adelaide, and I’m at the Australian Wine Research Institute (AWRI), tucked into the corner of a campus on the outskirts of the city. I’m here to meet senior sensory scientist Wes Pearson, who is busy at work in the preparation room of the institute’s sensory laboratory. This is a busy place: the AWRI run frequent sensory sessions, both with consumer and expert panels, where they try to understand why some people prefer some wines to others.
Pearson is juggling with a large array of vials, each of which contains individual chemical constituents of wines. He uses a special pipette to accurately spike a commercial red and white wine with a particular concentration of these chemicals, so that we can assess their sensory impact. This time we’re looking at a range of fault compounds, among others.
long one side of the preparation lab are small wooden windows. These each open into small sensory booths, where the panel members sit. The booths are isolated, and are a spotless white, with a small sink recessed into each desk. The idea is that the panel members should have no distractions at all, and be free to concentrate on the assessment task. The window opens from the next room, the glasses are passed in, and the panellists taste, marking their verdicts on a small tablet computer in front of them.
One of the samples that Pearson prepares is of a compound called beta-ionone, which contributes to the fruity aromas of some red wines. “I’m interested in whether or not you can smell this,” he says, pointing out that a proportion of people just don’t get it at all. I take a sniff of the doctored red wine, and immediately there’s a big blast of floral/raspberry aroma. I can smell beta-ionone. It turns out that there are quite a few compounds like this in wine that we differ in our ability to smell.
One of the most famous is rotundone. A few years back, the AWRI identified this as the compound responsible for the peppery character in red wines, especially cool-climate Syrah. It’s also responsible for the peppery aromas of peppercorns. Surprisingly, though, one in four of the population can’t smell it at all, even at heroic concentrations. It turns out that ionone and rotundone are just two of the growing band of wine-relevant aromas that we differ in our ability to detect.
The supertaster
The concept that we might all be living in rather different taste worlds dates back some 85 years. It was back in 1931 that Dr A.L. Fox, working at his laboratory in DuPont, made a momentous discovery.
He’d been synthesizing a novel chemical, and knocked over a vial full of it. As the dust flew into the air, his colleague remarked on how intensely bitter it tasted, while he couldn’t taste it at all. Health and safety rules were different then, and Fox ran round the building testing whether people could spot it: 60% could, while the remainder found it tasteless. The chemical was phenylthiocarbamide (PTC).
Propylthiouracil (PROP), which is closely related and a bit safer, is now used instead, and separates the population into groups of tasters and non-tasters. In the 1990s, researcher Dr Linda Bartoshuk coined the term ‘supertaster’ to describe a subset of tasters with heightened sensitivity to PROP/PTC. This became an active field of research, as researchers showed ‘supertasters’ seemed to have greater sensitivity to a broad range of different flavours, such as the burn of alcohol and the bitterness of coffee. Dr Bartoshuk describes them as living in a ‘neon’ taste world. While it was found that the different sensitivity to PROP was genetic, and depends on the variant of the TAS2R38 gene (which codes for a taste receptor) that people possess, there also seemed to be a correlation between supertasting and having more taste buds on the tongue. Could it be that supertasters make better wine tasters? Initially there was a lot of excitement in the wine world about PROP taster status, and whether different sets of consumers might prefer different types of wines. And some leading critics came out claiming to be supertasters.
In 2012, Dr John Hayes of Penn State University (USA) and Dr Gary Pickering of Brock University (Canada) did a study in which they looked at a group of wine consumers and compared them with a group of wine experts. They found that wine experts are more likely to be medium- or super-tasters than wine consumers. “If you have an innately superior ability to taste things, maybe you gravitate towards a field where that is rewarded,” speculates Dr Hayes.
But since then new research has emerged that has made the PROP account look simplistic. Humans have 25 bitter taste receptors, and TAS2R38, the one that detects PROP, is just one of them. There’s TAS2R31, which detects the bitterness of artificial sweeteners acesulfame-K (Ace-K) and saccharin. It’s possible to be highly sensitive to PROP bitterness and insensitive to saccharin bitterness, or vice versa. TAS2R31 is responsible for detecting the bitterness of quinine and also corresponds to grapefruit liking. And TAS2R3/4/5 can explain the bitterness that some people experience with alcohol.
“Bitterness is not a monolithic trait,” says Dr Hayes. “You can’t measure one thing and expect it to predict everything.”
There is also a difference in the way people experience astringency. The mainstream explanation for astringency perception is called the ‘delubrication hypothesis’. Saliva contains proteins that lubricate the mouth, and tannins bind to these proteins and cause them to precipitate out. This strips the lubrication and we sense this change as astringency. But people differ widely in their salivary flow rate. The more saliva, the less astringent we find tannins. Also, people differ in their ability to replenish salivary proteins. As a result, some people are much more sensitive to astringency. Dr Hayes says that a quarter of the population really struggle to replenish their salivary proteins. For this group, many red wines will be unpleasantly tannic.
Then there are specific anosmias: odours that some people just can’t smell. For example, the OR10G4 gene explains differences in responses to guaiacol, a smoky aroma found in wines suffering from smoke taint. The version of this gene that you have will predict how intensely you experience guaiacol, and those who experience it more intensely also like it less. Dr Hayes thinks that this gene might also explain the different reactions of people to 4-ethylphenol and 4-ethylguaiacol, the two main aroma impacts of Brettanomyces, but this work remains to be done. Variants in the OR2J3 gene influence how intensely people experience cis-3-hexenol, a grassy aroma. Variants in OR5A1 affect the way that the floral-smelling beta-ionone is experienced (roughly a quarter of the population are insensitive to the violet/floral aromas in some wines, and will probably like these wines less). And there’s the common smell blindness to rotundone, for which the gene is not yet known.
Another source of variation is in the enzymes present in the mouth, called glycosidases. Many aroma compounds in wine are found as inactive glycosides. This is when an aroma compound has a sugar molecule attached to it, and is no longer aromatic because it is now too heavy to be volatile. These glycosides are described as “locked away” aromas, and are more abundant in wine than the free volatiles themselves. Mango Parker, a researcher at the AWRI, has shown that people differ in the glycosidase enzymes found in their saliva. In a study, she found that 6 out of 11 panellists seemed to have enough of these enzymes to liberate significant aroma from wine once it was in their mouths. So variation in flavour perception is more complex than PROP taster status alone.
“Yes, the PROP story has probably been oversold to the public,” says Dr Hayes. “It's an easy, dramatic example that we really do live in different sensory worlds, but, no, it can't explain everything.” He adds, “the molecular genetics of PROP tasting are actually very clear, but critically, they appear to be totally unrelated to ‘supertasting’. In my view it seems very likely that PROP phenotype is capturing two totally separate sources of variance: receptor genetics (which we understand well) and overall amplitude/intensity (which we don't).”
Commercial applications
So should wine companies be taking this variation in flavour perception into account? “Yes, I think winemakers and producers should take individual differences into account,” says Dr Hayes. “Probably not for PROP per se, but definitely for astringency or rotundone peppery aroma. I don't know of any that are doing it in a formal systematic way from the biology side.” Dr Hayes suggests that even smaller producers would benefit from acknowledging that not all consumers are the same. “It is critical because philosophically that would imply there is not one, and cannot be one, platonic ideal for ‘the best wine’,” he says. “That is, those people who like sweet fruity wines may not be uneducated philistines to be sneered at by wine enthusiasts; rather, they may have a fundamentally different palate. And as capitalists, wine producers should be willing to meet the needs of these consumers, not just their winemaker friends and peers.”
Large wine companies have done extensive market research dividing consumers up into target segments (great examples would be the Wine Nation and Project Genome segmentation exercises carried out by Constellation Brands). But these latest findings offer a way to segment consumers along biological differences. Rather than offer one wine designed to appeal to an ‘average’ palate, it might be more effective to offer three that are specifically tuned to certain biological segments in the population.
“I think discrete segments probably exist, as they do for other food products,” says Dr Hayes. “But what I have spent the past eight years working on is trying to convince people that these segments are not merely idiosyncratic. Instead, they probably have a fundamental basis in biology. And if I’m right, this would mean we can move beyond segmenting people for single foods, and instead generalise across foods.” Dr Hayes also emphasises that while much of our liking for products such as wine, beer and cheeses seems to be learned rather than innate, that there are limits to how we can acquire tastes. “No amount of training can make up for fundamental biological differences.”
Back in the AWRI sensory lab, Wes Pearson takes me through some more doctored wines, and then we begin to talk about mousiness. This is an interesting wine fault, and it’s on the rise with the increasing number of non-intervention and natural wines. The main chemical responsible for the taint is 2-acetyltetrahydropyridine (ACTPY), which is not aromatic at wine pH, but begins to change pH once you get the wine in your mouth, to the point that you can smell the typical mousiness signature of caged mice and cracker biscuits. But there’s a twist – a third of the population just don’t sense this fault. I count myself unlucky: I love natural wines, but a proportion of them have mousiness. If only I was one of those lucky folk who didn’t get mousiness, then I could enjoy more of these wines.
